The present invention relates to an array type laser diode and, more particularly, to an array type laser diode in which a thermal stress generated when the diode is mounted on a substrate with a solder is reduced and its heat dissipation is improved.
A laser diode used in arrayed light-transmitting/receiving module applied to optical interconnection is an array type element in which a plurality of channels are integrated. This laser diode is used by fixing it on a silicon substrate by passive alignment mounting. The array type laser diode mounted on the silicon substrate is optically coupled to an optical fiber or optical waveguide.
Mainly two types of laser diodes are available, i.e., one using a p-type substrate and one using an n-type substrate. An active layer is buried close to the mounting surface. Striped electrodes are formed on the mounting surface to be parallel to the active layer. Usually, the laser diode is mounted on the silicon substrate through the mounting surface such that the active layer is located at the lower side.
Conventionally, when a p-type substrate array type laser diode 100 having this arrangement is to be mounted on a silicon substrate, only striped cathode electrodes 104 formed on the mounting surface immediately under the active layer are bonded to the silicon substrate with a solder, as shown in FIG. 6.
The mounting process will be described. Cathode electrodes and electrode pads are formed on the array type laser diode and the silicon substrate, respectively, such that striped cathode electrodes and the electrode pads formed on the silicon substrate surface coincide with each other when the laser diode is mounted on the silicon substrate. For bonding, solder layers are formed on the electrode pads of the silicon substrate in advance.
After that, the array type laser diode is positioned at a predetermined position on the silicon substrate at room temperature, and the entire silicon substrate is heated to fuse the solder layers, thereby bonding the electrode pads and cathode electrodes to each other. The structure is cooled to room temperature to solidify the fused solder. The electrode pads of the silicon substrate and the cathode electrodes are bonded to each other through the solder, thereby fixing the array type laser diode to the silicon substrate.
When the temperature decreases from the solder fusing temperature to room temperature, a thermal stress is generated on the solder bonding surface. This thermal stress concentrates on a region near the solder bonding surface. Hence, the thermal stress generated in the cathode electrodes is transmitted to the active layer arranged close to the cathode electrodes, to degrade the characteristics or reliability of the active layer.
The thermal stress acting on the active layer increases in accordance with an increase in the number of channels. Accordingly, the number of channels of the array type laser diode is limited to about 4. When a 12-channel module, which is the mainstream nowadays, is to be formed, a plurality of array type laser diodes are required. The mounting process accordingly includes a plurality of steps to increase the cost.
Heat generated when the laser diode is driven is transmitted to the substrate through the cathode electrodes on the bonding surface and the electrode pads on the substrate. Since the bonding surface is small, heat is not transmitted easily to provide poor heat dissipation. In order to decrease the thermal stress acting on the active layer and to improve heat dissipation, the bonding area between the laser diode and silicon substrate must be increased.
In order to operate the laser diode at a high speed of about several GHz, the bonding area must be decreased to reduce the junction capacitance. In this manner, from the plurality of viewpoints described above, contradictory requirements are imposed on the area of the bonding electrodes. Such requirements are conventionally difficult to satisfy simultaneously.
It is an object of the present invention to provide an array type laser diode in which a thermal stress acting on the active layer when bonding the laser diode to the substrate can be reduced and its heat dissipation in use is improved.
It is another object of the present invention to provide an array type laser diode which can operate at a high speed.
In order to achieve the above objects, according to the present invention, there is provided an array type laser diode comprising an active layer formed in a first substrate into a stripe shape to serve as an emission portion, a drive electrode formed on a mounting surface of the first substrate into a stripe shape to be close to the active layer, and bonding electrodes formed on the mounting surface of the first substrate to be physically separated from the drive electrode.